The development of new HIV-1 antivirals is dependent on our understanding of the individual steps in viral replication. Perhaps the least understood aspect of HIV-1 replication is the process of disassembly. Our preliminary data suggest that the viral protein Nef and the cellular factor cyclophilin A (CyPA) are involved in some aspect of disassembly. We have shown that HIV-1 viruses that have their own nef deleted and replaced by HIV-2 or SIV Nef "in trans" become resistant to cyclosporine A treatment. This suggests some interdependence between Nef and CyPA. It has been previously shown that Nef and CyPA can directly interact. We have confirmed this result in our own lab using Far Western blots and SELDI mass spectrometry. We consider whether the interactions between Nef and CyPA and their potential interactions with CA in the viral core are important for replication. Our hypothesis is: that interaction between Nef, CyPA and CA are important for enhancement of viral infectivity. Our approach is summarized in three specific aims: 1. Characterize the ability of HIV-2 and SIV Nef to induce cyclophilin A independence in HIV-1 virions using a multi-round infectivity assay. 2. Determine if the interaction between HIV-1 Nef and CyPA is relevant to enhancement of infectivity. 3. Determine if the HIV-1, HIV-2 or SIV Nef proteins can directly or indirectly bind to CA protein. A better understanding of the relationship between Nef and CyPA will provide critical details about this poorly characterized step in replication, which could provide an attractive new target for antiviral therapy.